The hydration of alkali metal silicate glasses is well known to the art. U.S. Pat. Nos. 3,498,802 and 3,498,803 provide fundamental discussions regarding the mechanism of the hydration reaction and indicate the wide range of glass compositions that can be sucessfully hydrated. Those patents also disclose the various property phenomena imparted to such glasses when water is incorporated into the structure thereof. Of especial interest is the low temperature thermoplasticity which the hydrated glasses can demonstrate.
Some hydrated glasses, however, exhibit such poor resistance to weathering and chemical attack that the surfaces thereof require immediate and continued protection from the ambient atmosphere lest the desirable thermoplastic behavior be lost. U.S. Pat. No. 3,811,853 takes advantage of that circumstance and describes the formation of products having compositions within defined limits of the alkali metal silicate system which will, under certain conditions, self-degrade in the ambient environment. Thus, self-degradation begins spontaneously after a weathering-resistant surface, previously applied to the articles, is purposely penetrated or removed, thereby exposing the interior of the articles to the ambient atmosphere.
An analysis of the above patents indicates that chemical durability and resistance to weathering (along with the degree of thermoplasticity exhibited by the hydrated body) are dependent upon the amount of water included in the glass structure. This appreciation led to research drawn to methods for controlling the amount of water which would be diffused into the glass during the hydration treatment.
One such process is disclosed in U.S. Pat. No. 3,912,481 which contemplates a two-step process. Thus, an alkali metal silicate glass is initially subjected to a saturated or near-saturated steam atmosphere at elevated temperatures, which step causes the essential saturation of the glass structure with water. Thereafter, the saturated glass is exposed to an environment of lower relative humidity such that the glass is dehydrated. This dehydration can be precisely controlled which permits the final water content of the glass to be carefully tailored.
A second such process is disclosed in U.S. Pat. No. 3,948,629 which involves a single-step, solution-hydration process. Thus, an alkali metal silicate glass is contacted with an aqueous acidic solution at elevated temperatures and pressures. Via regulation of the composition of the hydrating solution, the temperature, and the pressure, close control of the amount of water diffused into the glass structure is possible.
A third such process is disclosed in U.S. application Ser. No. 822,877, filed Aug. 8, 1977 in the names of Pierson and Tarcza. The method involves subjecting finely-dimensioned bodies of alkali silicate glass to a steam atmosphere of less than 50% relative humidity at very elevated temperatures. The low steam content of the hydrating environment leads to a relatively slow rate of water diffusion into the glass, thereby permitting excellent control of the amount of water taken up by the glass.
Each of those methods is operable within a broad range of starting glass compositions. Hence, on an anhydrous basis, operable glasses consist essentially, in mole percent on the oxide basis, of 3-25% Na.sub.2 O and/or K.sub.2 O and 50-95% SiO.sub.2, the sum of those components constituting at least 55 mole percent of the total composition. Useful optional ingredients noted in those disclosures included up to 25% Al.sub.2 O.sub.3, up to 20% BaO, up to 25% B.sub.2 O.sub.3, up to 35% MgO, up to 25% ZnO, up to 20% PbO, and up to 25% CaO. Each of those disclosures also describes generally the capability of the hydrated materials to be compression molded at relatively low temperatures.
U.S. Pat. No. 4,046,545 describes a method for producing glass components of optical quality from hydrated glasses having compositions within a defined range. The method comprehends utilizing the general hydration process described in U.S. Pat. No. 3,912,481, supra. Thus, a glass having an anhydrous composition consisting essentially, in mole percent on the oxide basis, of 70-82% SiO.sub.2, 10-17% Na.sub.2 O and/or K.sub.2 O, and 5-15% ZnO and/or PbO is hydrated in a saturated or near-saturated steam atmosphere at elevated temperatures and pressures. Thereafter, the glass is dehydrated through exposure thereof to an atmosphere of lower relative humidity, an inert gas being conventionally introduced into the environment throughout the dehydration period to maintain the total pressure greater than the vapor pressure of the hydrated glass. Customarily, the partially dehydratd glass will have a water content of about 3-8% by weight. The partially dehydrated glass is then molded in a conventional manner, the use of tungsten carbide die surfaces being noted as yielding optical quality surfaces. The securing of optical quality hydrated glass in the bulk of the articles is achieved by means of the above-described deyhdration technique which assures homogeneous distribution of water throughout the glass bulk.
The availability of optical glasses with a wide range of refractive index is essential for many applications. For example, the curvature of the fast objective lens can be reduced when a glass of high refractive index is utilized for that purpose. It is well-recognized in the optical glass art that the addition of lead to a glass normally raises the refractive index thereof. However, the inclusion of additional PbO in the glasses of U.S. Pat. No. 4,046,545, supra, to raise the refractive indices thereof, i.e., to values over 1.60, customarily resulted in the glasses developing a yellow coloration after hydration.